WHY DOES A NEMATODE HAVE AN INVARIANT CELL LINEAGE

C. elegans is renowned for its invariant embryogenesis and functions a s a major paradigm for a mode of development coupled to an invariant l ineage. Recent work, however, suggests that the embryogenesis of the n ematode is much more flexible than anticipated. The invariant premorph ogenetic stage is formed from variable earlier stages through a sortin g of cells. Cells do not act as individuals but already early in embry ogenesis a regionalization of the embryo occurs. Cells are diversified by a binary specification of 'abstract' blastomere (regional) identit ies. The determination of tissues may thus be a very late event. It ap pears that C. elegans, although assigning cell fates in an invariant l ineage pattern, uses the same strategies and mechanisms for embryogene sis as organisms with variable lineages.